1. Field of the Invention
The present invention relates to optical scanning devices, and more particularly, relates to optical scanning devices suitable for image forming apparatuses such as digital copiers, laser printers, and the like.
2. Description of the Related Art
Image formation using an optical scanning method is widely performed in image forming apparatuses such as digital copiers, laser printers, and the like.
A known optical scanning method capable of implementing high-speed image formation is, for example, a multiple-beam scanning method. Vertical cavity surface emitting lasers (VCSELs) have been increasingly used as a laser light source suitable for a scanning method.
Another type of laser light source using a multiple-beam scanning method includes an edge emitting type semiconductor laser array. Among semiconductor lasers, there is also a technology for combining beams using a compound prism using a plurality of edge emitting semiconductor lasers (EELs). With these technologies, only a few light-emitting elements can be arranged at the same time.
In contrast, with the vertical cavity surface emitting laser (VCSEL), tens to hundreds of laser light-emitting elements can be arrayed in a same plane where laser light is emitted and moreover, they can be individually modulated. Accordingly, with this technology, tens to hundreds of scanning lines can be simultaneously drawn. This makes it possible to fully exhibit a high-speed performance during image formation, which is an advantage of multiple-beam scanning.
However, a typical problem associated with a vertical cavity surface emitting laser is that light intensity dynamically changes when elements are driven (dynamic behavior). Such a dynamic behavior includes, for example, droop characteristic, rise time characteristic, and fall time characteristic. Related technology has been disclosed in, for example, Japanese Patent Application Laid-open No. 2006-332142 and Japanese Patent Application Laid-open No. 2008-213246.
It is known that this kind of phenomena observed in commonly used semiconductor lasers is caused by a change in threshold current because the light source element itself is heated by a current applied thereto or caused by the CR-time constant of the electric circuit. Image density varies due to these phenomena, resulting in the occurrence of poor image quality, such as uneven density, uneven color tone, and the like. Accordingly, a technology called an automatic power control (APC) to reduce such a problem has been developed.
The conventionally used edge emitting semiconductor laser (EEL) and the vertical cavity surface emitting laser differs in characteristics, such as a wavelength characteristics or a driving characteristic, due to their structural differences.
In particular, the driving characteristics significantly differ. In the edge emitting semiconductor laser (EEL), because mode hopping (wavelength hopping) occurs for an extremely short period of time when it is driven, the optical path length of the resonator changes due to heat. The gain function of a laser medium also changes due to a sudden change in characteristics immediately after a current is applied. When these changes occur, it is possible to jump up to the most advantageous mode of oscillation (large gain).
FIG. 1 is a diagram illustrating an example of observing mode hopping. The horizontal axis in FIG. 1 indicates a wavelength, and the vertical axis indicates elapsed time. FIG. 1 illustrates the optical response for each wavelength in a range of about 50 nanoseconds [ns] immediately after a driving current is applied.
Immediately after the driving current is applied, a mode on a short wavelength side (648.17 nanometers [nm]) rises. Then, a mode on a long wavelength side gradually become dominant (mode hopping), and the modes eventually come to be a single mode.
In FIG. 1, the gap between neighboring modes is 0.16 nm, i.e., about 0.2 nm, which is extremely small compared with the commonly used edge emitting semiconductor laser (EEL), which has a wavelength of 650 nm; therefore, there is no problem in terms of image forming characteristics. Specifically, this indicates that the stability of total optical output in all modes is relatively high with respect to any change in the state of the elements.
However, because vertical cavity surface emitting lasers (VCSEL) emit only one wavelength, no mode hopping theoretically occurs. There is a significant difference between the wavelength of neighboring modes, e.g., a difference in wavelength by a factor of 0.5 or 2. For example, a neighboring mode with respect to a vertical cavity surface emitting laser with a wavelength of 780 nm is 390 nm or 1,560 nm; the difference is extremely large, and, therefore, vertical cavity surface emitting lasers (VCSEL) do not oscillate because the gain of the laser medium cannot be obtained.
Accordingly, because vertical cavity surface emitting lasers (VCSEL) oscillates in a single mode whatever the circumstances, vertical cavity surface emitting lasers (VCSEL) are less flexible compared with the edge emitting semiconductor lasers (EEL); therefore, a stable optical output cannot be obtained.
In addition, a sudden change in temperature in the active layer of a semiconductor laser due to a current being applied causes a change in refractive index, which changes the state of optical confinement. Accordingly, a divergence angle (FFP: far field pattern) of a laser beam instantaneously varies; the FFP is small in a range, between a current-applied time t and a current-applied time 0, and then it becomes large over time. This variation commonly appears as a change in light intensity (rise time characteristic) in an optical system having an aperture.
In a scanning optical system, the effect of light intensity on a target surface becomes large, regardless of the system being an under-field type or an over-field type.
FIGS. 2A and 2B depict graphs that represent the dynamic behavior of unstable light intensity caused by the factors described above. The horizontal axis indicates the elapsed time from when a current-applied time is set to zero, and the vertical axis indicates observed light intensity. The dynamic behavior of the light intensity of the VCSEL is observed when a small current is applied. A case in which a current applied can be small is, for example, as follows:
(1) A case in which the number of light-emitting elements in the VCSEL is large.
(2) A case in which the sensitivity of a photosensitive element is high.
In a region where a small current is applied, FIG. 2A indicates that the optical intensity is low, and FIG. 2B indicates that the optical intensity is high. The difference Δ between light intensity (P2), obtained when the optical intensity is stable, and light intensity (P1) when the current-applied time is close to zero is Δ(a) % when the optical intensity is low and Δ(b) % when the optical intensity is high, as shown in FIGS. 2A and 2B, respectively. Δ is calculated using Equation 1 below:Δ=|(P1−P2)|/P2  (1)
When the difference between Δ(a) and Δ(b) is large, the light intensity varies in each optical scanning device, whereby a poor image is formed.
The problem of the dynamic behavior of the light intensity shown in FIGS. 2A and 2B described above has been dealt with a technology in which driving is electrically controlled. However, to deal with the typical problem of vertical cavity surface emitting laser (VCSEL), the above technology is inadequate.